1. Field of the Invention
The present invention relates to a medium conveyance apparatus which adopts a distributed control system.
2. Description of the Related Art
To control an image forming apparatus such as a printer or a copying machine, central control which uses a single CPU has conventionally been adopted. However, this control poses problems in that the concentration of control in a particular CPU increases its CPU load and increases the lengths of bundled lines routed to a load driver unit spaced apart from a control CPU board. To solve such problems, a distributed control system in which each control module constituting the image forming apparatus is divided into individual sub-CPUs is attracting a great deal of attention. In the distributed control system, a plurality of control modules which are arranged to be distributed operate in cooperation with each other while sending/receiving, for example, pieces of control information and commands.
In the field of robot arms, a distributed control system has already been proposed (Japanese Patent Laid-Open No. 2001-147706). The distributed control system is built using a plurality of sub-control CPUs for controlling a plurality of arm joint actuators, and a main control CPU for supervising their cooperative control. Control data for cooperative control is supplied from the main control CPU to the sub-control CPUs via a network in advance, and the sub-control CPUs drive the actuators, respectively, based on the control data in synchronization with a common clock signal from a clock supply unit.
Unfortunately, the technique described in Japanese Patent Laid-Open No. 2001-147706 poses the following problems. For example, because a robot arm etc. must have fast response characteristics, the modules are connected to each other via a high-speed network. The cooperative control includes, for example, feedback control implemented by allowing, for example, a distance measurement/detection control module which detects its distance from an object gripped by the arm and an actuator control module to work with each other. The distance measurement/detection control module notifies the main control CPU of the detected amount. The main control CPU supplies control data to the sub-control CPUs of each actuator control module. Moreover, the main control CPU notifies each sub-control CPU of a synchronizing clock. To obtain fast response characteristics in the feedback control, the overhead on the network is non-negligible. This makes a high-speed network necessary.
However, the cost rises upon applying a high-speed network to distributed control of the image forming apparatus. On the other hand, a plurality of conveyance rollers are provided in the conveyance path of a medium (paper sheet), and different sub-CPUs control these conveyance rollers for respective conveyance sections. This makes it necessary to transfer a medium among the plurality of sub-CPUs. If the plurality of sub-CPUs cannot obtain timing synchronization, the medium may suffer tension or looping (slack), leading to a paper jam. Nevertheless, a conveyance section in which the plurality of sub-CPUs may perform asynchronous conveyance control is also available, depending on the size of the medium.